Check for empty user agent to avoid notice.

Props sergej.mueller
fixes #22979


git-svn-id: http://core.svn.wordpress.org/trunk@24275 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Don't show both "update failed" and "update successful" messages when upgrading a theme fails due to connectivity problems.

Props SergeyBiryukov
fixes #23027


git-svn-id: http://core.svn.wordpress.org/trunk@24276 1a063a9b-81f0-0310-95a4-ce76da25c4cd